This invention relates generally to power plants having a pressure fluid source and more particularly to a closed loop hydraulic drill feed system.
For drill feed systems in use today, it is common practice to use a hydraulic cylinder with its associated hydraulic system to control feed system movement and force. Because most hydraulic cylinders used in this application exhaust more oil while retracting than they do while extending (unbalanced), the hydraulic circuits used are of the conventional open loop type. For these systems, pump discharge is supplied to a directional control valve which then directs the oil supply appropriately to extend or retract the hydraulic cylinder. In these circuits, oil discharged from the cylinder as a result of cylinder piston movement, returns first to the valve and then back to the system reservoir. Oil supplied to the pump in the first place comes directly from the system reservoir. Because the system reservoir is included in the pumping loop (at the intake of the pump) the system is called an open loop system.
It is evident that, for open loop systems, the characteristic of unequal flows is of little concern because the unbalance is accommodated by the system reservoir. It is this same characteristic, however, that has historically prevented unbalanced cylinders from operating in closed loop (with the reservoir separated from the main pumping loop) drill feed systems.
Previous attempts to operate unbalanced cylinders in closed loops have relied on various controls to replenish the loop, and exhaust oil from the loop, as required by cylinder movement. For example, when the cylinder is extending, the pump receives too little oil back from the cylinder. The use of a check valve to allow oil flow from the reservoir to the pump inlet in this replenishing mode is common practice. Also, when flow in the circuit is reversed and the cylinder is retracting, the cylinder supplies too much oil to the pump. Attempts to return the surplus return oil to the system reservoir have made use of pilot check valves or pilot controlled directional valves. These methods cannot, however, provide the precise position control and stable operation demanded by the drill feed system because these type valves tend to be either open or closed with no flow modulating capabilities.
FIG. 1 illustrates a conventional open loop cylinder feed system with a directional control valve controlling movement of a feed cylinder. The components included in the circuit are a reservoir assembly with a filter and check valves, a pump, the directional control valve, the feed cylinder (unbalanced) and an overcenter valve to provide load holding capabilities. For this circuit, unequal flows produced by cylinder movement are accommodated by the system reservoir. Some types of feed system pumps may even require that inlet oil be at some pressure higher than atmospheric pressure. Methods such as pressurizing the reservoir or boosting inlet oil by other means may be incorporated but the open loop concept remains the same. Supercharge pump pressurization is shown to demonstrate the technique. In this system it is necessary to precisely control the operation of both the directional control valve and pump flow to extend and retract the feed cylinder in an efficient manner. In the absence of automatic controls, the task of operating appropriately is left to the machine operator. It is evident also that the directional control valve contributes to total feed system efficiency loss in both directions of cylinder movement. Another limitation is that filtration capacity must be great enough to accommodate pump flow and pump surplus flow during cylinder retraction.
The foregoing illustrates limitations known to exist in present devices. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.